This invention relates to devices having an anode, a cathode, and an optoelectronic film between the anode and the cathode. The invention especially relates to such devices where the optoelectronic film comprises an organic optoelectronic material.
Optoelectronic devices such as photocells (e.g., photodetectors, photodiodes, photovoltaics) and electroluminescent (EL) elements may formed by sandwiching films comprising optoelectronic materials between electrodes. When an EL device is subjected to an applied voltage, holes injected from the anode and electrons injected from the cathode will combine in the optoelectronic material to form singlet excitons which can undergo radiative decay, liberating light. Conversely, in photocells, light that is incident upon the optoelectronic material is converted into electric current.
Optoelectronic devices have been made with certain inorganic and organic semi-conductors as the optoelectronic materials. In addition, the film comprising the optoelectronic material may comprise a plurality of layers based on materials of the desired properties. Constituent organic optoelectronic materials may be polymeric, as described by Kraft and coworkers in Angew. Chem. Int. Ed., Vol. 37, pp. 402-428, (1998), or monomeric, as described by Tang and VanSlyke in U.S. Pat. No. 4,885,211 and by Tang in Information Display, pp. 16-19, October, 1996. Other suitable materials include those disclosed in U.S. Pat. No. 5,708,130, and 5,728,801, W097/33193, U.S. patent application Ser. Nos. 09/123,271 and 09/289,344.
The anode is typically a transparent or semi-transparent conducting material, deposited on a transparent substrate, such as glass, so that light can escape from the EL element or so that the optoelectronic film can be exposed to light. Indium-tin-oxide (ITO) is generally the preferred anode material because of its excellent optical transparency and electrical conductivity. Because most organic materials have low electron affinity, efficient injection of electrons into them from the cathode is only possible when the cathode is a metal of low work function which can be deposited as pin-hole-free films by evaporation in high vacuum or by sputtering. Preferred metals are lithium, calcium and magnesium, as well as their alloys and blends with metals of higher work function. The use of low work function metals in EL elements leads to higher EL efficiency but also environmental instability as these metals are known to be extremely sensitive to oxygen and moisture in ambient air. Indeed, EL elements with calcium cathodes have been reported to lose 90% of their efficiency in 37 seconds in a highly humid environment according to Sheats, et al. in Science, Vol. 273, (1996), pp. 884-888. Magnesium is sometimes seen as a compromise choice yet its stability in EL elements still leaves much to be desired as Tang and VanSlyke (U.S. Pat. No. 4,885,211) had shown that efficiency of these elements may drop by more than an order of magnitude in a matter of hours when exposed to an ambience with a relative humidity of 20% or higher due to cathode corrosion. In addition to the cathodes, polymer films in EL or photocell devices must also be protected from ambient oxygen and moisture as the injection of charge carriers generates highly sensitive chemical species: radical anions formed by injection of electrons and radical cations formed by injection of holes into the polymer film are readily destroyed by oxygen and water.
There is clearly a need for an effective packaging scheme to protect EL elements and photocell elements from ambient environment if they are to be used in commercial displays. This need has been recognized and several packaging approaches have been reported.
WO97/46052 teaches the use of a sheet of low melting metal alloys bonded onto the cathode of an EL element. Since the alloy layer is in direct contact with the cathode and, indeed, serves as the wiring contact, this approach is not suitable for EL elements with patterned cathodes, which is the preferred method for creating dot-matrix displays.
Another approach (see EP 777,281, and WO 97/16053) involves coating the cathode first with an organic film which is, in turn, coated with layers of metals, metal oxides, inorganic oxides, or, inorganic fluorides and the like. The problem with this approach is the application of the organic coating onto the cathode. Once an EL element is formed, no part of it may be exposed to moisture, organic solvent, oxygen, and elevated temperatures without causing damage. Thus the application of an organic coating to a formed EL element, possible in principle, would be extremely difficult to accomplish without damaging the EL element in some way.
Yet another approach described in JP 7014675 involves co-forming films which are mixtures of inorganic fluorides and oxides and plasma-polymerized poly-p-xylylene. The inventor acknowledged the inferior barrier (to oxygen and moisture) properties of the polymer vs inorganic materials because of the presence of macro defects. Therefore, diluting the beneficial barrier properties of the inorganic materials with poly-p-xylylene can hardly be an advantage. Furthermore, organic emitting materials are readily damaged by intense ultraviolet light inherent to the plasma generation process. Even if this packaging approach could protect the cathodes from oxygen and moisture in ambient environment, it is likely to cause irreversible damage to the optoelectronic properties of the organic material.
A fourth approach involves sealing the flange of a metal or glass lid with a UV-curable adhesive onto the glass substrate of the EL element in vacuum or in an atmosphere of very dry nitrogen as described by Nakada and Tohma in Display Devices, 1998, pp. 29-32. The dimensions of the lid are chosen such that there is a gap between the inner surface of the lid and the cathode. The adhesive film must provide adhesion between the cover and element and barrier to ingress of moisture and oxygen. Since adhesion and barrier properties result from different chemical designs, it""s unlikely for an adhesive to perform well in both functions. Material selection would be a compromise. It is also critical that the adhesive film be free of voids and pinholes and the adhesive be free of volatile organic compounds, dissolved gas, and moisture which would be otherwise trapped in the sealed compartment and will eventually cause device deterioration.
It is clear that all of the known approaches have limitations. It is the object of this invention to provide a simple, yet effective, packaging scheme free of said limitations.